The Neurotoxic Effect of 13,19-Didesmethyl and 13-Desmethyl Spirolide C Phycotoxins Is Mainly Mediated by Nicotinic Rather Than Muscarinic Acetylcholine Receptors

Abstract

Spirolides are a large family of lipophilic marine toxins produced by dinoflagellates that have been detected in contaminated shellfish. Among them, 13,19-didesmethyl and 13-desmethyl spirolide C phycotoxins are widely distributed and their mode of action needs to be clearly defined. In order to further characterize the pharmacological profiles of these phycotoxins on various nicotinic acetylcholine receptor (nAChR) subtypes and to examine whether they act on muscarinic receptors (mAChRs), functional electrophysiological studies and competition binding experiments have been performed. While 13-desmethyl spirolide C interacted efficiently with sub-nanomolar affinities and low selectivity with muscular and neuronal nAChRs, 13,19-didesmethyl spirolide C was more selective of muscular and homopentameric α7 receptors and recognized only weakly neuronal heteropentameric receptors, especially the α4β2 subtype. Thus, the presence of an additional methyl group on the tetrahydropyran ring significantly modified the pharmacological profile of 13-desmethyl spirolide C by notably increasing its affinity on certain neuronal nAChRs. Structural explanations of this selectivity difference are proposed, based on molecular docking experiments modeling different spirolide-receptor complexes. In addition, the 2 spirolides interacted only with low micromolar affinities with the 5 mAChRs, highlighting that the toxicity of the spirolide C analogs is mainly due to their high inhibition potency on various peripheral and central nAChRs and not to their low ability to interact with mAChR subtypes.

Synthesis and biology of cyclic imine toxins, an emerging class of potent, globally distributed marine toxins

“…It has been found that, on isolated frog or mouse neuromuscular preparations, gymnodimine A induces a reversible time- and concentration-dependent blockage of twitch tension responses evoked by nerve stimulation without affecting directly-elicited muscle twitches. 94 Similar effects were later observed with 13-desmethyl spirolide C 95 and pinnatoxin F, with the latter toxin causing a complete abolition of the rat hemidiaphragm action potential responses evoked by nerve stimulation, while no effect was observed after an electrical stimulation. 96 In addition, gymnodimine A blocks the miniature endplate potential in frog and mouse isolated neuromuscular preparations.…”

Marine natural products

“…909 Electrophysiological and competiton binding experiments have identied that 13-desmethyl spirolide C is a potent but relatively non-selective ligand of nAChRs while 13,19-didesmethyl spirolide C is more selective of the muscular-type receptor. 910 Both MNPs interacted weakly with muscarinic AChRs. Further investigation of the unusual occurrence of tetrodotoxin in New Zealand collections of the nudibranch Pleurobranchaea maculata has led to the metabolite being detected in mucin cells, the mantle, gonad tissue and the digestive gland of the nudibranch as well as in the larvae and eggs but not in the gelatinous egg cases.…”

Cyclic imine toxins from dinoflagellates: a growing family of potent antagonists of the nicotinic acetylcholine receptors

“…), 13‐desmethyl spirolide C and 13,19‐didesmethyl spirolide C (Aráoz et al . ), 20‐methyl spirolide G (Couesnon et al . ), and pinnatoxins E, F, and G (Hellyer et al .…”

“…), 13‐desmethyl spirolide C and 13,19‐didesmethyl spirolide C (Aráoz et al . ), and pinnatoxins E, F, and G (Hellyer et al . ), either reduced or completely blocked (depending on concentration) the amplitude of spontaneous miniature endplate potentials, an effect suggesting that the toxins impeded the interaction of ACh quanta with endplate nAChRs.…”